Impact winter
From Freepedia
An impact winter is caused by debris from an impact of an asteroid or comet obscuring the sun. These, and other types of unnatural winters, are often the subject of science fiction novels and/or short stories. So far in recorded history, an impact winter has never occurred, and one is not expected for at least 100 years because there is no known object on a collision course with earth that's large enough to generate one. It is widely believed that the dinosaurs perished during an impact winter which occurred 65 million years ago. The impact winter may have been caused by an asteroid about 15 kilometers wide landing on the Yucatan peninsula. It is theorized that an impact winter would cause the extinction of the human race.
On March 16, 2880, the asteroid (29075) 1950 DA has a one in 300 chance of impacting Earth. Such an impact would cause an impact winter.
